This study investigates growth of the neural retina in adult goldfish. We know that neurons are added as the retina enlarges; photoreceptors (the rods) are especially interesting because they are added in proportionately greater numbers than the other retinal cell. My goal is to find out where new rods come from and how they are assimilated into an already functioning retina. I use quantitative and experimental anatomical techniques, especially 3H-thymidine autoradiography which labels dividing cells and their progeny. Preliminary evidence suggests that new rods are generated by a unique precursor cell located in the outer nuclear layer, among the mature rod nuclei. All other new retinal neurons are formed by germinal cells located in a restricted zone at the margin of the retina. If it is true that the developmental origin of rods is different from other retinal neurons, even from the other principal type of photoreceptor, the cone, it may help to explain the somewhat puzzling difference in susceptibility of rods and cones to environmentally-induced and genetic lesions. In the rat retina, for example, it is the rods that are most affected by damaging levels of photic stimulation. It is again the rods, and not the cones, which degenerate in patients with retinitis pigmentosa and in the animal models for that disease (the rd mutant in mice and the RCS rat). Though the cell lineages of photoreceptors are not known in mammals, or in any other species, the peculiarities of rod genesis in fish retinas suggest that such studies may be useful for understanding the cell biology of photoreceptors.